How a Jet Engine Works: Startup and Operation

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Air Compression

The primary function of jet engines is to provide controllable thrust to the aircraft. Photo Credit: Leo Papandreou

As is clear from the term, the function of a compressor is to simply compress the intake air and increase its density relative to the density of “outside air”. As an aircraft climbs through the air and gains altitude, air density decreases.

Decreased density means fewer molecules of air entering the jet engine at a given airspeed. Jets use compressors to compensate for this loss of density and attain higher efficiency ratios in jet engines.

Fuel Combustion Essentials

After the compressor assembly comes the combustion chamber. Its fundamental purpose is to sustain the ignited mixture. Fuel from the injection nozzles, and compressed air from the compressor, are combined here for ignition.

The combustion chamber must be able to contain a regulated flame and ensure simultaneous combustion of the fuel/air mixture. Efficient burning of the mixture is vital to the performance of a jet aircraft’s engines.

The Engine Turbine

Hot, expanding gases from the combustion chamber collide with the turbine assembly next. The purpose of a turbine in a jet’s engines is to mechanically harvest the energy of these speeding gases. In this case, speeding gases fall on the turbine and cause it to rotate. The turbine in-turn rotates the compressor, airplane propeller (turboprop) or the fan (turbofan) and other airplane accessories driven by the jet engine, such as the fuel, oil and hydraulic pumps (aircraft specific).

Exhaust System in Jets

The purpose of the exhaust system is to expel the exhaust gases in an efficient manner. These exhaust gases in turbojet engines typically provide jet thrust. The exhaust system must effectively direct the exiting gases at the proper speed, in order to be considered productive.

Direction of exhaust gases exiting the engine plays a vital role in determining the thrust line for that specific engine. Moreover, the fuselage structured in close proximity must be kept safe from the extreme properties of these gases exiting jet engines. The speed of these gases must be regulated to provide optimum thrust levels.

To summarize, all engines for jet aircraft operate on the Brayton cycle, which includes induction, compression, combustion, and exhaust.

Jet engines in airplanes provide propulsion in the form of thrust and are the most vital component of an airplane. Modern-day engines have been modified over time to attain higher levels of efficiency.

Currently, more research is underway regarding combustion chambers, their role in fuel saving and how their modification can help reduce atmospheric pollution.

Resources

S. M. Rizvi. Basic Airframe Precis. PIA Training Centre.

Oxford Aviation Services. Joint Aviation Authorities Airline Transport Pilot’s License Theoretical Knowledge Manual. (2001). Accessed January 6, 2012.

NASA Glenn Research Center. The Beginner’s Guide to Propulsion. Accessed January 6, 2012.

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